1. Field of the Invention
The present invention relates to a method of tracking satellite signals for Global Positioning System (GPS) receivers.
2. Description of the Invention
Positioning systems currently in service utilizing artificial satellites include a so-called Global Positioning System (GPS). This positioning system, as the name implies, will cover the entire surface of the Globe by a total of 24 geodetic satellites when all of them are operational on six orbits, approximately 20,200 km above the Earth, which will accommodate four geodetic satellites each. Each geodetic satellite transmits a GPS signal containing navigation data for reckoning to the Earth using a spread-spectrum system. The reckoning is carried out on the ground, on the sea as well as in the air by receiving GPS signals from a plurality of geodetic satellites, for example, by receiving GPS signals from three geodetic satellites for two-dimensional positioning and that from four geodetic satellites for three-dimensional positioning. In this way, based upon the navigation data contained in the GPS signal from each geodetic satellite, position information on the receiving point such as a latitude, longitude and altitude thereof can be reckoned in real time.
This GPS system was originally developed for U.S. military use, however, a part of the GPS signal (C/A code) has been made available for civil use. Therefore, it is possible to build navigation systems for automobiles, ships and aircraft by using the GPS signal.
A GPS receiver used for a vehicle-mounted navigation equipment starts a search operation to recapture the GPS satellite when the vehicle enters a tunnel, for example, and the reception of the GPS signals from the GPS satellites is interrupted for more than a specified period of time (for example, one minute).
The GPS satellites are not geostationary, so that the satellite signal receiving frequency may be shifted by the Doppler effect. Therefore, as shown in FIG. 1, in order to search for satellite signals, it has been required to change a search frequency in succession over a maximum frequency range of the Doppler frequency shift being centered on a center frequency (1575.42 Mhz) of a satellite signal.
The satellite signals, as mentioned above, are transmitted by the spread-spectrum system. Therefore, the reception of the satellite signal must be initiated by locking a phased-lock loop (PLL) circuit of the GPS receiver on the exact receiving frequency of the satellite signal at every stepwise change-over of the search frequency. Upon successful locking of the PLL circuit, the spread-spectrum signal is despread to receive the GPS signal. This means that even when the satellite signal has arrived at the GPS receiver, it is not able to receive the GPS signal without any delay.
Referring to FIG. 1, for example, suppose the vehicle comes out of the tunnel at time t1 and that an actual receiving frequency of the satellite signal at that time is fr, the PLL circuit of the GPS receiver will lock on the receiving frequency fr at time t2, which corresponds to eighth stepwise search frequency change after the time t1, thus permitting the GPS receiver to recapture the GPS satellite.
Therefore, when the satellite signal is not received, the conventional GPS receiver cannot discriminate a difference between a state of no reception caused by the incompletion of locking the PLL circuit on the receiving frequency or the failure of despreading for the received signal and a state of no reception caused by the lack of satellite signals themselves as the vehicle enters a tunnel and the like. Because of this ambiguity, the conventional GPS receiver is not capable of dealing with the interruption of the receiving signal, such as when the vehicle is in the tunnel, in a proper way, so that it takes a considerable time for the GPS receiver to recapture the GPS satellite after the vehicle passing through the tunnel, causing a problem in positioning.
It is therefore an object of this invention to eliminate the above drawback and to provide a satellite signal tracking method for a vehicle-mounted GPS receiver having a plurality of receiving channels, through which a GPS satellite can be recaptured in the shortest possible time when the satellite signal returns to normal after the interruption has occurred by such an obstacle as tunnel.